This invention relates generally to combustors, and more specifically to mixers used for enhancing fuel/air mixing in gas turbine engine combustors.
Modern day emphasis on minimizing the production and discharge of gases that contribute to smog and to other undesirable environmental conditions, particularly those gases that are emitted from gas turbine engines, have led to different combustor designs that have been developed in an effort to reduce the production and discharge of such undesirable combustion product components. Other factors that influence combustor design are the desires of users of gas turbine engines for efficient, low cost operation, which translates into a need for reduced fuel consumption while at the same time maintaining or even increasing engine output. As a consequence, important design criteria for aircraft gas turbine engine combustion systems include provision for high combustion temperatures, in order to provide high thermal efficiency under a variety of engine operating conditions, as well as the minimization of undesirable combustion conditions that contribute to the emission of particulates, and to the emission of undesirable gases, and to the emission of combustion products that are precursors to the formation of photochemical smog.
Various governmental regulatory bodies have established emission limits for acceptable levels of unburned hydrocarbons (HC), carbon monoxide (CO), and oxides of nitrogen (NOx), which have been identified as the primary contributors to the generation of undesirable atmospheric conditions. Therefore, different combustor designs have been developed to meet those criteria. For example, one way in which the problem of minimizing the emission of undesirable gas turbine engine combustion products has been attacked is the provision of staged combustion. In that arrangement, a combustor is provided in which a first stage burner is utilized for low speed and low power conditions to more closely control the character of the combustion products. A combination of first stage and second stage burners is provided for higher power outlet conditions while attempting to maintain the combustion products within the emissions limits. It will be appreciated that balancing the operation of the first and second stage burners to allow efficient thermal operation of the engine, while simultaneously minimizing the production of undesirable combustion products, is difficult to achieve. In that regard, operating at low combustion temperatures to lower the emissions of NOx, can also result in incomplete or partially incomplete combustion, which can lead to the production of excessive amounts of HC and CO, in addition to producing lower power output and lower thermal efficiency. High combustion temperature, on the other hand, although improving thermal efficiency and lowering the amount of HC and CO, often results in a higher output of NOx. In the art, one of the ways in which production of undesirable combustion product components in gas turbine engine combustors is minimized over the engine operating regime is by using a staged combustion system using primary and secondary fuel injection ports.
Another way that has been proposed to minimize the production of those undesirable combustion product components is to provide for more effective intermixing of the injected fuel and the combustion air. In that regard, numerous mixer designs have been proposed over the years to improve the mixing of the fuel and air. In this way, burning occurs uniformly over the entire mixture and reduces the level of HC and CO that result from incomplete combustion.
One mixer design that has been utilized is known as a twin annular premixing swirler (TAPS), which is disclosed in the following U.S. Pat. Nos.: 6,354,072; 6,363,726; 6,367,262; 6,381,964; 6,389,815; 6,418,726; 6,453,660; 6,484,489; and, 6,865,889. It will be understood that the TAPS mixer assembly includes a pilot mixer which is supplied with fuel during the entire engine operating cycle and a main mixer which is supplied with fuel only during increased power conditions of the engine operating cycle. Improvements in the main mixer of the assembly during high power conditions (i.e., take-off and climb) are disclosed in patent applications having Ser. Nos. 11/188,596, 11/188,598, and 11/188,470.
The mixers have swirler assemblies that swirl the air passing through them to promote mixing of air with fuel prior to combustion. The swirler assemblies used in the combustors are complex structures having axial, radial or conical swirlers or a combination of them. In the past, conventional manufacturing methods have been used to fabricate mixers having swirler components that are assembled or joined together using known methods to form the swirler assemblies. For example, in some mixers with complex vanes, individual vanes are first machined and then brazed into an assembly. Investment casting methods have been used in the past in producing some combustor swirlers. Other swirlers have been machined from raw stock. Electro-discharge machining (EDM) has been used as a means of machining the vanes in the swirlers.
Conventional combustor components such as, for example, mixers, are generally expensive to fabricate and/or repair because the conventional mixer designs include a complex assembly and joining of several components. More specifically, the use of braze joints can increase the time needed to fabricate such mixers and can also complicate the fabrication process for any of several reasons, including: the need for an adequate region to allow for braze alloy placement; the need for minimizing unwanted braze alloy flow; the need for an acceptable inspection technique to verify braze quality; and, the necessity of having several braze alloys available in order to prevent the re-melting of previous braze joints. Moreover, numerous braze joints may result in several braze runs, which may weaken the parent material of the component. The presence of numerous braze joints can undesirably increase the weight and manufacturing cost of the mixer assemblies.
Complexities of the swirler geometries and the associated difficulties in the machining and normal wear of the tools such as the EDM electrodes during the machining process, cause significant manufacturing variability in the mixer assemblies. Such manufacturing variability in the mixer assemblies may lead to undesirable aerodynamic flow variability in the mixers and adversely impact the aerodynamic performance of the combustor.
Thus, there is a need to provide a gas turbine engine combustor mixer using a manufacturing method with reduced variability. Further, it is desirable to have mixers with complex geometries and swirler arrangements having a unitary construction to reduce dimensional variations from manufacturing to improve operability and reduce emissions over the engine's operating envelope and to reduce costs. It is desirable to have a method of manufacturing a mixer having complex three dimensional geometries in a unitary construction.